The present invention is based on a semiconductor laser having a branched cavity layer.
Such a semiconductor laser is disclosed in Electronics Letters, Feb. 15, 1990, Vol. 26, No. 4, pages 243-244. It is monolithically integrated on an n-doped indium phosphide substrate. The underside of the substrate is called the base surface. Above a plane that is coplanar with the base surface, there extends a cavity layer of indium gallium arsenide phosphide (InGaAsP). It is branched and, when seen from the top, has the shape of a "Y". The top view surface of the cavity layer may also have a different shape, for example the shape of a cross. Significant is that the cavity layer is contiguous. This can be described in the sense of a topological definition in that the top view surface of the cavity layer can be considered to be a "singly contiguous region" since it is not composed of several pieces, but of a single piece having a single edge. The cavity layer lies on the planar surface of an n-doped buffer layer of indium phosphide that extends above the indium phosphide substrate.
Further layers are provided above the cavity layer. Above the buffer layer, these layers and the cavity layer form a mesa which has been produced by etching. In the plane that is coplanar with the base surface, the mesa as well as the cavity layer have a Y shape.
Such a laser with a branched cavity layer is provided according to the above-mentioned publication as an electrically controllable light source for optical communications transmission systems. In particular, if its metal layer extending above the cavity layer is subdivided into several electrodes so that cavity layer regions are created which can be controlled by means of different operating currents, such a laser is distinguished by its emission wavelength being tunable over a very wide wavelength range. This characteristic is of major importance for use as an electrically controllable light source as advocated in the publication.
Bistable components are gaining increasing significance for optical transmission. "Bistable" here means that, if actuated in the same manner, that is, with the same current or the same voltage, the component may assume two different states, that is, it emits light of a low or high intensity or light of a first wavelength or light of a second wavelength, depending on the manner in which the actuation state under observation has been realized.
Bistable components are suitable for use as optical switches or memories in wavelength multiplex switching systems and in optical data processing systems.
The publication IEEE Photonics Technology Letters, Vol. 2, No. 9, September 1990, pages 623-625, discloses a DFB [distributed feedback] semiconductor laser composed of two segments and operated as a wavelength bistable memory. It can be set and reset within 450 picoseconds by means of electrical pulses. The DFB semiconductor laser in each case emits light at one of two wavelengths which are spaced from one another by 0.9 nm.